Problem adding memory

I have been trying to add more memory to my system but not having any luck. I currently have 2 sticks of pc3200/512mb and want to add another 1gig stick but am having problems with it working. I have to press re-set after starting my computer in order for it to boot. I have tried removing the existing memory and only use the the 1gig stick but get the same results. I also tried a high-density stick with the same results, was able to swap that for a low density stick that was supposed to be compatible with all mobo's. Same results. Tried the same thing a year ago with another 512mb stick, same results. The 2 sticks that work great are both different manufacturers, so doesn't seem to be a problem in that regards.
Here are my specs:
p4 2.4 gig processor
via p4pb 400-l mobo
via apollo px400 chipset (vt8754 & vt8235)
ati 9600pro video card

All Mb's have specific configurations as to which slots are to be used with different numbers of sticks of Ram. Also it is best to use the recommended (tested)brand /type of Ram .
Go to the Website and confirm this info
It is never a good idea to mix different sticks , and generic ram is like shooting dice.